EXIT IN THE EMULSION POLYMERIZATION OF VINYL-ACETATE

The following model is proposed to explain the independence of polymer ization rate on monomer concentration which is often observed in the e mulsion polymerization of vinyl acetate: that radical loss is by trans fer to a monomeric species which is very slow to propagate and whose r adical activity is lost by exit and termination, either in the aqueous phase or when it enters a particle containing a growing radical. Sinc e the transfer step is rate-determining, the rate of this process is p roportional to monomer concentration, which cancels the dependence on monomer concentration in the overall polymerization rate expression; t his model also predicts that the radical loss rate coefficient should be either k(tr)C(p) or 2k(tr)C(p), depending on which is the actual fa te of the desorbed radical (where k(tr) is the rate coefficient for tr ansfer to monomer and C-p is the concentration of monomer in the parti cles). Experimental radical loss rates, determined directly by gamma-r adiolysis relaxation studies, are quantitatively in accord with the hy pothesis.